This invention relates to the recovery of Pt from gas turbine engine components in conjunction with the removal of Pt-containing coatings therefrom during repair and reworking.
Many advanced gas turbine engine components, especially turbine blades, are coated with platinum modified diffusion aluminide coatings (PtAl). These coatings offer superior environmental protection in oxidation and Type I hot corrosion conditions within a turbine engine. These coatings are also employed as bond coatings beneath physically vapor deposited (PVD) thermal barrier coatings.
The Pt present in PtAl coatings is most often deposited by electroplating. To develop the required PtAl chemistry and structure, about 0.5-0.8 grams of Pt are electroplated onto relatively smaller turbine blades, while up to on the order of 1.5 grams of Pt may be electroplated onto larger blades. After plating, the Pt is incorporated into the coating by diffusion, with the final composition of the predominant coating phase being (Ni,Pt)Al.
A PtAl coating may be removed from a blade if the coating itself or some other feature of the blade does not meet the engineering or quality requirements for the part. In such a case the coating is stripped, the part reworked and then recoated with PtAl. Turbine blades are also routinely stripped of PtAl coatings after engine operation to enable inspection and repair of the turbine blades.
Stripping of PtAl coatings is accomplished in a variety of manners. Most often the coatings are removed by acid stripping using mineral acids such as hydrochloric, phosphoric, nitric, and mixtures of these acids. The acids react with the coating and dissolve some of the coating constituents, especially Ni. After the reaction, a thin, loosely adherent, black film residue comprising Pt, aluminum oxides and heavy metal oxides of various elements from the substrate material is left behind on the blade. After stripping a number of parts, the stripping solutions become ineffective and must be discarded. Most often the acids are neutralized, the metals chemically precipitated out, and the precipitate filtered from the solution. The precipitate, although it contains minor amounts of Pt, is disposed of as solid waste in a landfill.
Residue has been removed from stripped turbine blades by mechanical methods such as wet blasting or vibratory finishing. Traditionally a xe2x80x9ccuttingxe2x80x9d type media has been used in vibratory finishing machines to remove residue. Cutting media consist of abrasive aluminum oxide particles in a soft binder. The binder breaks down, releasing aluminum oxide particles into the vibratory finish machine. Large volumes of sludge are produced. The residue is thus contaminated and diluted by the abrasive media as it breaks down, such that the Pt is no longer economically recoverable. Overflow from wet blast or vibratory finishing machines used to clean blades is treated in a wastewater system, and the solids, although they contain Pt, are disposed of as waste.
In situations where PtAl coatings are removed mechanically by abrasive grit blasting, rather than chemically, Pt-bearing debris from grit blasting is filtered from the process air and is disposed of as solid waste.
By one of the above processes, the Pt from the PtAl stripping operation finds its way into the final waste stream of a turbine blade repair plant. Many high volume operations feed into the final waste stream in a manufacturing plant and dilute the concentration of Pt in the solid waste to the point where it is not economically viable to recover the precious metal.
The invention is directed to a method for recovering Pt from a Pt-bearing coating on a gas turbine engine component. The component is contacted with a chemical stripping agent to dissolve away constituents of the coating and produce a Pt-bearing film of acid oxidation products at the surface of the component. The film is removed from the component to produce a Pt-rich residue containing at least about 10% Pt and the residue is refined to recover Pt therefrom.
Other features of the invention will be in part apparent and in part pointed out hereinbelow.